[README] Add links to the screencasts

.gitignore

@@ -1,2 +1 @@
-voronoi-*
 *.ppm

README.md

@@ -1,6 +1,6 @@
 # Voronoi with OpenGL using Depth Buffer
 
-![thumbnail](./thumbnail.png)
+![thumbnail](./thumbnails/thumbnail.png)
 
 ## Quick Start
 
@@ -8,3 +8,9 @@
 $ ./build.sh
 $ ./voronoi-opengl
 ```
+
+## Screencasts
+
+[![voronoi-01](./thumbnails/voronoi-01.png)](https://www.youtube.com/watch?v=kT-Mz87-HcQ)
+
+[![voronoi-02](./thumbnails/voronoi-02.png)](https://www.youtube.com/watch?v=qIOAz7M1syw)